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“Women who smoke face going through the menopause at least a year earlier than non-smokers,” the Daily Mail has reported.

The news story was based on new research that combined the findings of previous studies in a bid to find out whether smoking affects the age at which a woman will naturally experience menopause. By bringing together the results of 11 studies researchers found that smoking was significantly associated with early menopause, which smokers experienced approximately one year earlier on average.

Earlier natural menopause is of interest in several fields of medicine as it has been associated with a lower risk of some diseases (for example, breast cancer) as well as an increased risk of several other diseases and earlier death.

As the researchers themselves have noted, this study does have some limitations and the link between smoking and early menopause is still speculative. While this particular link needs further testing, there are many proven health benefits from stopping smoking.

Where did the story come from?

The study was carried out by researchers from Hunan Normal University and Central South University, China; Tulane University, US, and the University of Hong Kong, Hong Kong. Funding was provided by the Natural Science Foundation of China, the NSFC-Canadian Institutes of Health Research and Joint Health Research Initiative Proposal and the University of Hong Kong start-up fund.

The study was published in the peer reviewed journal Menopause, the journal of the North American Menopause Society.

This story was covered in the Daily Mirror and the Daily Mail. The coverage of results from the study is accurate. The Daily Mail also included opinions from experts and data from other reports.

What kind of research was this?

This was a systematic review and meta-analysis that aimed to evaluate how smoking affects the age at which natural menopause will occur. Natural menopause occurs when the ovaries naturally decrease their production of the sex hormones, such as oestrogen, and cause the menstrual cycle to cease. It differs from surgical and induced menopause, which occurs if the ovaries are removed or damaged by treatments such as radiotherapy for cancer.

The researchers say this is an area of interest because the age of natural menopause is associated with both positive and negative health outcomes. For example, earlier natural menopause is associated with a lower risk of some diseases (for example, breast cancer) but also with an increased risk of other diseases and earlier death. The researchers say that so far the results published on the relationship between smoking status and menopause age have been conflicting.

The researchers report that previous reviews in this area had not included a meta-analysis, a statistical technique used to pool and analyse the results of several studies. This technique can be an appropriate way to summarise study results as long as the studies themselves are similar in nature.

What did the research involve?

The researchers searched the Medline and Google Scholar databases to identify all relevant studies in English published between 1977 and 2009 that had examined the association between smoking and age at natural menopause.

The results from each study were combined into two meta-analyses. One combined studies looking at the average age at menopause for smokers and non-smokers, and the other included studies looking at each group’s chances of experiencing menopause before a certain cut-off age. In these studies, early menopause was defined as menopause occurring before 50 years of age, although 51 years of age was used for one study. The researchers then calculated the odds of early menopause in these studies.

The researchers also correctly used accepted methods to analyse the studies to look at how uniform they were (which affects how accurately their results can be combined), and performed tests to see if their combined results were being influenced by the results of any one study.

They also looked at all the studies to see if the results were influenced by a publication bias, a phenomenon where studies that show a significant difference are more likely to be published than ones that show no difference.

What were the basic results?

After analysing the literature search results, the researchers included 11 studies in their meta-analyses. All of the studies were observational studies (seven cross-sectional studies, three case-control studies and one cohort study).

Five studies, with a total of 43,155 participants, provided data on smoking status (smoking versus non-smoking) and then reported the number of participants with certain age ranges at natural menopause, such as under or over 50 years old at the time of menopause. Early menopause was defined as menopause occurring before 50 years of age in four studies, and before 51 years in one. These were termed 'dichotomous studies', meaning they split participants into categories.

The other six studies, including 6,010 participants, gave the mean age of natural menopause in the smoking and non-smoking groups. These were termed ‘continuous studies’ as they looked at time of menopause across a continuous age range.

The combined result for all the included dichotomous studies showed an association between smoking and early menopause, with non-smokers 26% less likely to have a menopause before 50 years (odds ratio [OR] 0.74, 95% confidence interval [CI] 0.60 to 0.91).

Similar results were obtained after the researchers adjusted for non-uniformity of the studies (OR 0.67, 95% CI 0.61 to 0.73). They recalculated their results to express the chance of early menopause for smokers, and their results suggested that smoking could increase your risk of early menopause by between 35% and 49%.

The combined results for all the included continuous studies suggested that menopause occurs approximately one year earlier in smokers (weighted mean difference of smokers versus non-smokers of -1.12 years, 95% CI -1.80 to -0.44). After adjusting for non-uniformity of the studies the results were similar (weighted mean difference -0.90, 95% CI -1.58 to -0.21).

The researchers found no evidence of publication bias.

How did the researchers interpret the results?

The researchers conclude their results ‘suggest that smoking is a significant independent factor for early age at natural menopause’.

Conclusion

This study systematically searched for studies and combined their results to see if there is an association between smoking and age at natural menopause. They found that smoking was significantly associated with early menopause. As the researchers note, this study had some limitations, and the link between smoking and early menopause is still speculative and needs further testing. These include:

Many of the studies they included in the analysis did not provide detailed information on smoking habits and age at natural menopause. For example, none of the studies reported the duration or level of participants’ smoking, and only three provided definitions of menopause status.

Cohort studies that follow people over time would be the ideal study for testing this theory but the researchers only found one available for inclusion in their analysis. Other study types are more prone to bias from other unmeasured factors that might also be having an effect.

Although it is not certain if or how stopping smoking will affect the age at which menopause occurs, there are many proven health benefits from stopping smoking.